The invention relates to a fountain composition for use in offset printing, and to a fountain solution comprising the composition.
In offset printing, aluminum plates which have a hydrophilic surface (non-printing areas) and carry an oleophilic layer in the image areas (printing areas) are clamped on an impression cylinder which is frequently provided with a nickel plating. The hydrophilic and oleophilic areas are in the same plane (planographic printing). Besides the printing ink (greasy), a so-called fountain solution is required for printing. The fountain solution keeps the image-free areas in a hydrophilic state, such that ink is only accepted by the oleophilic image areas and is transferred to the paper via the rubber blanket. In order to improve the printing result, substances which are to optimize, for example, viscosity, pH value and compatibility with the printing ink and prevent faults which may be caused by regionally different water qualities, by the paper or by microorganisms, are customarily added to the fountain solution.
The most favorable pH range for printing is between 4.7 and 5.3 and is normally maintained by buffer substances such as, for example, citric acid/citrate. U.S. Pat. No. 4,548,645 also discloses salts obtained from polycarboxylic acids and organic bases, for example amines, which are employed for this purpose. The hitherto used fountain compositions, however, produce corrosion phenomena in the printing machines, which occur, in particular, on the plate and blanket cylinders provided with a nickel plating and may give rise to considerable machine down-times and repair costs.
Many attempts have therefore been made to reduce or prevent corrosion on machine parts, in particular, on the nickel plated cylinders of printing machines.
In DE-A 35 36 485 a fountain composition is described which, with the view to reducing corrosion phenomena, contains copper ions, in addition to the usual components including water, substances producing an increase in viscosity, buffers based on citric acid, surfactants and fungicides. Using this fountain composition, nickel erosion can be reduced as long as copper ions are present.
This means that in these fountain compositions the copper, being an electrochemically noble metal, is deposited on the generally used baser aluminum support and this may lead to scumming phenomena during printing; moreover, the copper ions which are only present in small quantities are removed from the fountain solution and are thus lost for the protection against corrosion.
EP-A-0 108 883 describes a process for avoiding corrosion on printing machine cylinders, in which 1 H-benzotriazole is added to the fountain composition as a corrosion inhibitor. In this process, a pH value of 5.5 or higher should be maintained.